A spark plug for use in a combustion apparatus, such as an internal combustion engine, includes, for example, an insulator having an axial bore extending in the axial direction; a center electrode provided at the forward side of the axial bore; a terminal electrode provided at the rear side of the axial bore and electrically connected to the center electrode via an electrically conductive glass seal; a tubular metallic shell attached externally to the insulator; and a ground electrode joined to a forward end portion of the metallic shell. Application of high voltage to the center electrode generates spark discharge across the gap formed between the center electrode and the ground electrode; as a result, fuel gas is ignited.
In order to further improve ignition performance, there has been proposed an ignition plug (high-frequency plasma ignition plug) which generates plasma discharge through application of high-frequency power in place of high voltage to the gap (refer to, for example, Japanese Patent Application Laid-Open (kokai) No. S51-77719 “Patent Document 1”).